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Leg curls, which are most commonly performed on a machine, force you to bend your knees against resistance. The muscles that make up your butt -- gluteus maximus, gluteus minimus and gluteus medius -- are involved in controlling stability and movement at the hip joint. Therefore, during most types of leg curls, the butt muscles are not recruited and therefore won’t increase in size.

The Leg Curl Exercise

Most gyms offer multiple types of leg curl machines. The most common include the seated and front-lying machines, both of which involve selecting the load you’ll lift by placing a pin in a stack of weight plates. The seated machine places you in a sitting position. Your legs begin extended out in front of you, with your calves resting atop a leg pad. Bend your knees to pull the leg pad down and back toward your glutes, moving the pad as far as you can, and then straighten your knees to complete the rep. The front-lying machine is similar, except that you perform the exercise from a face-down lying position. Your legs begin extended and a leg pad rests atop the back of your calves. Bend your knees to bring the leg pad toward your glutes and then extend your legs to lower the pad to starting position.

Muscles Worked

The muscle group that leg curls primarily work is your hamstrings, which is a collection of three muscles at the back of your thighs. If you’re performing leg curls and feel it burning in your butt, it’s likely because the hamstrings originate up at the backside of your pelvis. When performed at an appropriate volume, leg curls can effectively increase the size of your hamstrings. Also helping out during leg curls are your gastrocnemius and soleus, which are the two major muscles in your calves.

Butt Involvement

Your gluteus maximus is recruited during the leg curl on an exercise ball. Lie on your back on the floor with your lower legs resting atop an exercise ball. Lift your hips up off the floor and hold that position as you bend your knees to roll the ball toward your glutes. Extend your legs to roll the ball back out to finish the rep. While your hamstrings are still the primary muscle developed, your gluteus maximus contracts to hold your hips off the floor. However, the glutes are acting as a stabilizer and contracting isometrically, so leg curls on an exercise ball still won’t provide enough of a stimulus to overload and build size in your butt.

Targeting the Butt

If you’re interested in building the size of your butt, instead of leg curls, incorporate deadlifts, lunges, squats and step-ups into your workout regimen. Work your glutes two days per week and select two to four of these exercises to include in each workout. Complete each exercise for three to five sets of eight to 20 reps.

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About the Author

Kim Nunley has been screenwriting and working as an online health and fitness writer since 2005. She’s had multiple short screenplays produced and her feature scripts have placed at the Austin Film Festival. Prior to writing full-time, she worked as a strength coach, athletic coach and college instructor. She holds a master's degree in kinesiology from California State University, Fullerton.